The present invention relates generally to power control circuits, and particularly to dimming circuits for lighting and other applications.
Dimming circuits have used SCRs to "chop off" or drop to zero potential a sinusoidal input wave during each half-cycle in manipulation of energy delivered to, for example, a lighting device. The smooth contoured sinusoidal alternating current power source is dropped to zero potential at selected phase angles, once each half-cycle, to selectably control the magnitude of energy applied to a load. By suddenly dropping the potential presented to the load, however, the cyclic wave form applied to the load is not properly characterized as a sinusoidal wave form. In deviating from a generally sinusoidal power wave form, such dimming functions have found deficiencies. Such dimming methods tend to have little control in the sudden shutoff phase and have required inductors to better control, i.e., extend, ramp time. More particularly, when suddenly dropping the potential of the potential applied to the load, the sudden change or step function in voltage and current flow produces undesirable consequences. Use of an inductor as a solution to step-voltage conditions introduces undesirable radio frequency interference. Furthermore, the basic "chop off" nature of the dimming function undesirably vibrates light filaments. This filament vibration causes both audible and RF interference and is desirably avoided.
More recently, use of insulated gate bipolar transistors IGBTs has been introduced in dimming functions. U.S. Pat. No. 4,633,161 issued Dec. 30, 1986 to Callahan shows an improved inductorless phase control dimmer power stage with semi-conductor controlled voltage rise time. The Callahan configuration only attempts to electronically simulate the prior SCR with inductor chop off mode of operation, but with improved ramp control during shutoff periods.
The background portion of the Callahan patent covers the history of dimming techniques including one approach to the "chokeless" dimmer as a high wattage power transistor operating in a pure linear mode. FIG. 2 of Callahan illustrates the output of a high wattage power transistor operating in a pure linear mode, however, FIG. 2 of Callahan illustrates an "amplitude clamping" function. Amplitude attenuation by clamping, while superior over phase control amplitude chopping, still introduces some degree of harmonics into the load circuit.
Callahan discusses some heat dissipating issues associated with such linear dimmers, and states that as much as ten times more heat must be dissipated from a linear dimmer relative to that of conventional phase control dimmers. In FIGS. 3A and 3B of Callahan, Callahan proposes pulse width modulation wave forms for application to the load as a solution to heat dissipation issues. FIG. 3B shows the result of introducing an inductor in series with the pulse width modulated wave form prior to application to a load. The use of an inductor, however, to produce such a synthesis of an amplitude-modulated sinusoidal wave form undesirably produces radio frequency interference.
Thus, there remains a need for an "inductorless" dimmer circuit having acceptable rise and fall times, but without excess heat dissipation requirements relative to prior phase control dimmer units. In this manner, the advantages of an inductorless dimmer function are achieved, but without the associated heat dissipation problems. The subject matter of the present invention provides such a dimmer control function.